The present invention relates to a method for coating a base coating (or base coat) paint and clear coating (or clear coat) paintto, particularly, motor cars by a wet-on-wet technique, and a base coating paint used therefor. In particular, the present invention relates to a coating method capable of remarkably improving the coating workability, and a base coating paint used therefor.
Recently, various curing systems for clear coating paints have been proposed. They include, for example, curing systems containing a hydroxyl group (including a blocked hydroxyl group), a carboxyl group (including a blocked carboxyl group wherein the hydroxyl group of the carboxyl group is blocked), a phosphoric acid group (including a blocked phosphoric acid group wherein the hydroxyl group of the phosphoric acid group is blocked) or an acid anhydride group; curing systems containing such a group as described above and also a silyl group (including a hydrolyzable silyl group blocked with a hydrolyzable group) and/or an epoxy group; curing systems containing an acetoacetyl group and a vinyl ether group or a vinyl thioether group (hereinafter referred to simply as a vinyl (thio)ether group); curing systems containing a vinyl (thio) ether group and a carboxyl group or a silyl group; curing systems containing an alicyclic epoxy group and a silyl group; and curing systems containing a silyl group or an alicyclic epoxy group singly. In such curing systems, a curing catalyst is usually used so as to accelerate the curing reaction of the functional groups.
In the wet-on-wet technique, the clear coating paint and the base coating paint are separately stored in storage vessels until immediately before the coating thereof, and these paints are separately transferred to a coating apparatus such as a nozzle or spraying means for the wet-on-wet coating. However, as the strage time of a clear coating paint comprising a functional group-containing oligomer or polymer (hereinafter referred to as "oligomer" collectively) and also the curing catalyst becomes longer, the clear coating paint begins to cure in the presence of such a curing catalyst and gradually thickened before the wet-on-wet coating is carried out. Therefore, when two or more kinds of functional groups are involved, the oligomers having different groups must be separated from each other and also when the curing catalyst is to be incorporated, it should be incorporated into only one kind of the oligomer to prevent the curing reaction. Even when an oligomer having only one kind of a functional group is used, it must be separated from the curing catalyst to prevent the curing reaction. When the paint is stored in the form of two or more liquids and they are mixed together immediately before the wet-on-wet coating is carried out, the flow rate and amount of each liquid must be controlled to prepare the clear coating paint, which makes the mixing device complicated and, therefore, makes the cost high for the apparatus.